OBJECTIVES: Investigation on the heme-free isolated subunits of hemoglobin has revealed that under certain conditions the folding of the system is reversible up to its quaternary structure after complete unfolding in 6M Guanidine-HCl. Also the subunits can be fragmented into large polypeptides that are in a partial state of folding. Comparative study of the physico-chemical characteristics of the fragments, of the entire subunits, and of mixtures of derivatives of the two kinds of subunits should give information on the mechanism of protein folding and assembly.